The present invention is directed, in general, to wireless networks and, more specifically, to an adaptive digital pre-distortion correction circuit for use in an RF transmitter.
Every wireless network base station has a RF power amplifier for transmitting voice and/or data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, pagers, and the like) and a receiver for receiving voice and/or data signals from the mobile units. The design of an RF power amplifier (PA) for digital radio systems is controlled by two overriding criteria: 1) The RF power amplifier should transmit sufficient RF output power to serve the cell site of the base station in which it is installed, but should also use the minimum amount of DC power in doing so; and 2) The adjacent channel power (ACP) noise (distortion) should be under certain limits (mask), that are usually defined in a standard (i.e., ACP profile).
In most cases, these two criteria are contradictory. ACP noise results from no-linear effects, such as over-driving the power amplifier into its nonlinear region (clipping). Spurious spectral components are introduced when a signal peak is sufficiently large to saturate an RF amplifier in the transmitter. In order to meet the ACP profile, the RF transmitters in wireless networks in which digital signals have high peak-to-mean ratios, such as CDMA and multi-carrier systems, are frequently xe2x80x9cbacked offxe2x80x9d from full power (or peak power) to avoid operating the transmitter in non-linear conditions. In these digital systems that have high peak-to-mean signal ratios, the RF power amplifier thus needs a considerable amount of power xe2x80x9cheadroomxe2x80x9d to accommodate the peak power. For example, RF power amplifiers in some CDMA systems need more than 10 dB of headroom space to protect the peak CDMA signal power from clipping. Unfortunately, leaving this much overhead significantly reduces the power efficiency of the RF power amplifier. This increases the DC power consumption, the base transceiver station cooling requirements, the overall system volume, weight, and cost.
For a particular digital radio system, such as cellular CDMA or TDMA, the transmitter ACP profile is defined in the system standard. Generally speaking, the actual ACP profile of an RF power amplifier is not the same as the ACP profile required by the standard. The power amplifier ACP profile is determined more or less by the power amplifier device characteristics, operating modes, and signal behaviors. For example, the out-of-band spurious components generated from a CDMA signal appear like white noise: the power density does not change significantly with frequency. However, the ACP profile defined in, for example, the IS95 CDMA system standard does not require a constant spurious power density over different frequencies. The whole frequency spectrum is divided into a few blocks and the standard ACP profile changes significantly from one block to the next.
This may lead to situations in which the power amplifier output power level is dictated by the ACP noise at few frequency points where the standard ACP profile appears the most stringent. However, there may still be relatively large ACP noise margins at many other frequencies. In a sense, the power amplifier ACP noise is not optimized to make full usage of the ACP profile under the applicable standard. The excess ACP noise margin at most frequencies is not utilized.
There is therefore a need in the art for improved wireless networks that use more efficient RE power amplifiers. In particular, there is a need for improved RE power amplifiers that can operate more closely to full power in systems having high peak-to-mean digital signal ratios. More particularly, there is a need for RF power controllers that make RF power amplifiers more efficient by utilizing the available ACP noise margins under the applicable standard ACP profile.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a pre-distortion adjustment circuit for use in an RF transmitter that optimizes the ACP profile of an RF power amplifier to fully use the ACP profile under the applicable RF communication standard. The present invention pre-distorts the RF signal so that the actual output ACP profile appears similar to, if not the same as, the ACP profile under the standard. The pre-distortion required is determined based on information extracted from the input signal, the output signal, and the standard ACP profile. Thus, the present invention allows significant overdrive of the power amplifier while still maintaining its ACP noise under the standard ACP profile.
Accordingly, in an exemplary embodiment of the present invention, there is provided, for use in an RF transmitter having an RF power amplifier required to transmit an RF output signal within selected limits of an adjacent channel power (ACP) profile specified for the RF transmitter, a pre-distortion adjustment circuit comprising: 1) input sampling means, coupled to an input of a transmit path of the RF transmitter, capable of capturing input samples from a digital input baseband signal, the input samples comprising a first input sample of amplitude X; 2) output sampling means, coupled to an output of the transmit path, capable of capturing output samples of a digital output baseband signal derived from the RF output signal, wherein a first output sample corresponds to the first input sample; and 3) processing means capable of determining from the first input sample and the first output sample a pre-distortion adjustment value capable of adjusting an amplitude of the digital input baseband signal prior to amplification by the RF power amplifier without causing the RF output signal to exceed the selected limits of the ACP profile.
According to one embodiment of the present invention, the specified limits of the ACP profile are stored in a memory associated with the processing means.
According to another embodiment of the present invention, the specified limits are specified at discrete frequency points.
According to still another embodiment of the present invention, the processing means applies the pre-distortion adjustment value to a subsequently received input sample of amplitude X.
According to yet another embodiment of the present invention, the processing means is capable of determining if the amplitude X is sufficiently small to ensure that an amplification distortion caused by the RF power amplifier is negligibly small and, in response to the determination, is capable of determining a scaling factor for the output samples.
According to a further embodiment of the present invention, the processing means scales subsequently received input samples of the digital input baseband signal according to a value of the scaling factor.
According to a still further embodiment of the present invention, the processing means adjusting an amplitude of the scaled digital input baseband signal.
According to a yet further embodiment of the present invention, the processing means modifies a selected subsequently received input sample according to a value of the scaling factor without regard to an amplitude of the selected subsequently received input sample.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.